The invention is based on a priority application EP03003635.4 which is hereby incorporated by reference.
The present invention refers to a pump energy source for providing pump energy to an optical transmission system transmitting an optical signal along an optical fiber, in particular an optical transmission system in which a beam of said pump energy is introduced to said optical fiber so that said beam of said pump energy copropagates with said optical signal.
The present invention further refers to a method of providing pump energy to an optical transmission system for transmitting an optical signal within an optical fiber, in particular an optical transmission system in which a beam of said pump energy is introduced to said optical fiber so that said beam of said pump energy copropagates with said optical signal.
The present invention moreover refers to an optical transmission system comprising one or more spans of optical fiber for transmitting an optical signal along said optical fiber, further comprising a pump energy source for providing pump energy.
Pump energy sources of the above mentioned type are widely used in optical transmission systems that comprise an optical fiber acting as a wave guide for guiding an optical signal. Since optical signals are subject to attenuation while travelling along an optical fiber, it is desirable to amplify said optical signals.
State of the art optical amplifiers are for example erbium-doped fiber amplifiers (EDFA) and Raman amplifiers the latter of which utilize the well-known stimulated Raman scattering (SRS) effect. Both of them do not require a conversion of the optical signal to the electric domain prior to amplification. In general, Raman amplifiers have the advantage that optical signal amplification is achieved directly in an ordinary optical fiber, whereas EDFA systems require e.g. erbium-doped optical fiber segments to effect an amplification.
Raman amplifiers can be set up in two different configurations: forward pumping configuration, in which a beam of pump energy copropagates with the optical signal to be amplified, and backward pumping configuration, in which a beam of pump energy counterpropagates with the optical signal to be amplified. Copropagating means that both the pump energy and the optical signal to be amplified are travelling in the same direction, whereas counterpropagating describes a state in which said pump energy and said optical signal to be amplified are travelling in opposite directions.
Forward pumping is considered to be a promising technique and can also be combined with backward pumping thus improving an optical signal-to-noise ratio (OSNR) and reducing double-Raleigh scattering.
However, present day pump energy sources such as common diode lasers have a comparatively high relative intensity noise (RIN), that is, undesired amplitude fluctuations of an output beam intensity. For most laser types, said RIN has a 1/f-characteristic which means that maximum RIN is at low modulation frequencies f. For higher frequencies, the RIN is limited by white noise which at the same time defines the minimum RIN.
Said RIN is transferred to the optical signal while said optical signal is being amplified by Raman amplification, especially in a forward pumping configuration, because the Raman scattering process on which said Raman amplification is based is nearly instantaneous. I.e. even high frequency fluctuations of said pump energy source do affect the amplification of said optical signal which is highly undesirable.